This invention relates generally to lithographic printing plate blanks and particularly to a type of lithographic printing plate blank which consists of a radiation transparent transfer sheet, coated on one side with a oleophilic material, and a receptor sheet having a lithographic hydrophilic surface, and in which a laser is applied through the transfer sheet to cause coating material on the transfer sheet to blow off onto the lithographic surface of the receptor sheet in a selected image pattern. The plate blank thus processed is thereafter inked and used for printing in a conventional manner.
For this laser processing the lithographic surface of the receptor sheet is placed in at least close proximity to coated surface of the transfer sheet; the laser beam is scanned over the uncoated side of the radiation transparent transfer sheet and is modulated for causing blow off of the coating in a selected pattern. Suitable laser scanning apparatus for this purpose is illustrated by U.S. Pat. No. 3,816,659 for Scanning Apparatus issued June 11, 1974, in the name of the present inventor.
It is known to transfer an ink like substance from one surface to another using laser radiation applied through a radiation transparent sheet to cause selected portions of the ink like material coated on the sheet to blow off onto an adjacent receptor sheet. Such a technique is decribed in U.S. Pat. No. 3,745,586, July 10, 1973 in the name of R. S. Braudy which relates to non-impact writing and in which the image thus produced is the final printed image desired. It is also previously known to use a lithographic surface as the receptor surface and to have the material transferred thereto by laser irradiation of an oleophilic or ink receptive material so that the resultant plate is a lithographic printing plate which is thereafter inked and applied to reproduce multiple printed copies in a conventional manner.
In the prior art, as illustrated by the aforementioned U.S. Pat. No. 3,745,586, it has been felt necessary to space the receptor sheet from the transfer sheet for the transfer step in order to provide an exit space for the gases generated by the combustion which accomplishes the transfer. If the gaseous combustion products do not disperse or do not disperse fast enough, they force the sheets apart or otherwise affect the transfer of coating material so that the resolution of the transfer image is non-uniform or totally degraded. However, it has been demonstrated in the art that a suitably well defined image can be formed with the transfer and receptor sheets in intimate contact for the transfer step if: (1) the surface of the receptor sheet is grained or otherwise slightly roughened so as to provide passages for the gaseous combustion products to disperse from between the sheets and (2) the critical surfaces of the transfer and receptor sheets (ie. the surface areas at which the transfer occurs) are maintained uniformly in contact. If the sheets are not kept in uniform contact, if gas buildup causes the sheets to bulge apart in a localized area, the resolution of the transfer image will be distored at that point. Since the uncoated surface of the transfer sheet must be exposed to the beam of laser radiation which initiates the combustion transfer, it is a problem to find suitable means for holding the sheets together. The transfer and receptor sheets may be stored separately and assembled into a composite plate on the laser scanning apparatus, for example by making the transfer sheet larger than the receptor sheet and using a vacuum holddown, but for commercial and practical purposes it is more desirable to have the two sheets assembled and held together as a composite blank during manufacture. This would simplify packaging, handling and storing and would mean that the person processing the blank would not have to assemble separate sheets, but would simply mount a single composite blank on the laser scanning apparatus.